1. Field of the Invention
The present invention is located in the field of semiconductor optical components used for optical transmission or the processing of optical data. It relates more particularly to all optical components comprising an interferometer structure using phase variations. To simplify the explanation of the invention, we shall refer hereinafter only to a Mach-Zehnder type interferometer structure. However, as has been stated, the object of the invention also extends, in its scope, to all other structures comprising a phase modulator.
One of the main problems that are generally sought to be resolved in integrated optics consists in making polarization-independent devices, namely devices that are independent of light polarization (namely TE, TM polarization).
2. Description of the Prior Art
Standard Mach-Zehnder type modulators, made with semiconductor materials, comprise stacks of layers on a substrate. This stack is made by successive deposits of a lower cladding layer, an active layer forming the active waveguide of the component and an upper cladding layer. The layers are deposited by a standard method of epitaxy. The first modulators in the prior art used substrates based on lithium niobate (LiNbO.sub.3), which is a material highly dependent on the light polarization. This material was then replaced by indium phosphide InP. In general, the active waveguide is constituted by a non-doped single quaternary material, of the InGaAsP type for example.
The phase variation .DELTA.j, in a phase modulator, is proportional to the product of the variation of the refraction index .DELTA.n in the active waveguide multiplied by the length L of this active guide. In fact, it is governed by the following relationship (1): EQU .DELTA.j=(2.pi./.lambda.).times..DELTA.n.times.L (1)
where .lambda. represents the operating wavelength. The operating wavelength is defined as the wavelength of the carrier wave used to ensure the working of the component. The variation of the refraction index .DELTA.n is furthermore equal to the product S.times..DELTA.E where S represents the sensitivity of the component and .DELTA.E the variation of electrical field applied to ensure its operation. .DELTA.E is also written as .DELTA.V/e, where .DELTA.V represents the variation of the control voltage and e the thickness of the active waveguide. The relationship (1) can therefore be written as follows: EQU .DELTA.j=(2.pi./.lambda.).times.S.times.(.DELTA.V/e).times.L (2).
From this relationship, it is deduced that, to reduce the control voltage in a phase modulator, it is necessary to increase the length. Now the length is a function of the capacitance C. Indeed, if the length L is increased, the capacitance C of the component is increased because C is given by the following relationship: EQU C=(.epsilon.WL)/e (3),
where W represents the length of the active waveguide and .epsilon. the permittivity. The relationship (2) therefore becomes: EQU .DELTA.j=(2.pi./.lambda.).times.(1/ (.epsilon.W)).times.S.times.C.times..DELTA.V (4).
This relationship shows that for a fixed capacitance C (hence for a fixed passband), the phase variation is independent of the length and the thickness of the active layer. Consequently, to reduce the control voltage .DELTA.V, it is necessary to increase the sensitivity S.
The aim of the invention therefore is to make a phase modulator that is insensitive to the light polarization and has high sensitivity, so that this phase modulator can be controlled with a low voltage.
The curves of FIG. 1, which represent the sensitivity S (=.DELTA.n/.DELTA.E) of a standard phase modulator as a function of the operating wavelength .lambda., illustrate the main phenomena brought into play during the working of this component.
A first phenomenon brought into play concerns the Franz-Keldysh effect. This phenomenon appears when an electrical field is applied to the component. It is expressed by a variation of absorption that occurs in the active waveguide. This phenomenon is always accompanied by a variation in the refraction index .DELTA.n in the waveguide. This is why this phenomenon, throughout the rest of the description, is known as the Franz-Keldysh Electro-Refraction effect referenced FKER. This effect depends on the polarization of light. It is illustrated by the two curves, plotted in unbroken lines and dashed lines respectively, for the TE and TM modes of light polarization.
In the example illustrated in FIG. 1, the photoluminescence wavelength .lambda.g of the material used to form the active waveguide is 1250 nm. This means that if this material is illuminated with a wavelength below this photoluminescence wavelength .lambda.g, the material becomes absorbent. By contrast, if it is illuminated with a wavelength greater than .lambda.g, the material becomes transparent. The wavelengths in the range that can be used to make the component work must therefore be greater than the photoluminescence wavelength .lambda.g of the material constituting the active guide.
The curves of the sensitivity S of the component as a function of the operating wavelength .lambda., for the FKER effect, show that the closer the operating wavelength is to the photoluminescence wavelength .lambda.g of the material, the greater the increase in sensitivity S and the more dependent the FKER effect is on the TE and TM light polarization (the sensitivity S.sub.FK/TM is greater than the sensitivity S.sub.FK/TE).
A second effect brought into play in this structure relates to the Pockels effect. This effect is expressed by a variation of the refraction index .DELTA.n of the material constituting the active waveguide when an electrical field is applied. This phenomenon is independent of the operating wavelength .lambda. in the range of wavelengths commonly used for the operation of this structure. Furthermore, for the section generally used, this Pockels effect is always zero for the TM light polarization.
By contrast, for the TE light polarization, this phenomenon is highly dependent on the orientation of the guides with respect to that of the substrate. Indeed, if the guides are made according to an orientation that forms an angle .alpha.=0.degree. with respect to the crystalline orientation of the substrate, a TE-related positive Pockels effect appears and is added to the first FKER effect described here above. By contrast, if this angle .alpha. is equal to 90.degree., the TE-related Pockels effect is negative and is deducted from the first effect FKER. When this angle is equal to 45.degree., the Pockels effect gets cancelled.
Given that, when the sensitivity S of a standard Mach-Zehnder component increases, the FKER effect becomes polarization sensitive, a known approach consists of the use of the Pockels effect to compensate for the anisotropy that appears between the TE and TM modes of light polarization.
Thus, given that the FKER effect for the TE mode is smaller than the FKER effect for the TM mode of light polarization (see curves of FIG. 1), to enable compensation for this difference, it is necessary to add the index variation .DELTA.n due to the TE-related Pockels effect. Thus, it is necessary to make waveguides in such a way that they are oriented in a direction forming an angle of 0 to 45.degree. with respect to the crystalline direction of the substrate.
FIG. 2 illustrates a prior art phase modulator 50 of this kind with a Mach-Zehnder type structure of this kind, wherein the waveguides form an angle to the crystalline direction of the substrate. The two guiding arms, referenced 51 and 52, of the phase modulator 50 are indeed made along the directions [0,0,1], while the substrate 53 is oriented along the direction [0,-1,1] (which is the generally used section). In this example, the guiding arms are therefore oriented in such a way that the direction of propagation of light is close to 45.degree. with respect to the cleaved faces 56 of the substrate. This orientation of the guide implies the making of bends in the active waveguide, thus complicating the manufacturing technology.
These devices however have several drawbacks. The Pockels effect can compensate for the FKER effect only up to a certain sensitivity Sp which still remains very low. Indeed, when the angle to be formed between the crystalline direction of the waveguide and that of the substrate is zero, the Pockels effect (Sp) is the maximum and it is therefore no longer possible to compensate for the FKER effect beyond. This compensation limit implies a limiting of the sensitivity of the component (this limit is represented by a double arrow, referenced L in FIG. 1, and corresponding to the maximum sensitivity Sp due to the Pockels effect).
Now the problem that it is sought to resolve is to make a light-polarization-independent phase modulator having a high sensitivity S so that it can respond to a low control voltage.
The present invention makes it possible to overcome the above-mentioned drawbacks because it proposes a light-polarization-insensitive semiconductor phase modulator having a very high sensitivity S.